Photovoltaic solar cells are semiconductor devices that convert light energy to useful electrical energy. Generally speaking, a solar cell typically includes a silicon wafer having regions of n-type doping and regions of p-type doping. Solar radiation generates mobile electrons and holes that migrate to the different regions and create a voltage differential between the two regions. Patterned metal layers on the silicon wafer are required to conduct the generated electricity out of the cell. The metallization of one cell is soldered to a separate piece which is then soldered to the next cell. Many such cells soldered together are then encapsulated between a front glass which faces the sun and a backing material which provides mechanical protection as well as a moisture barrier. Encapsulating materials include ethylene-vinyl acetate (EVA) and silicone polymers.
In solar cells based on crystalline silicon, the silicon itself is a major cost. In fact, the silicon alone can account for 20% to 30% of the cost of the finished product. Thus, there has been an increasing effort to develop thinner silicon cells. In recent years, the average thickness has decreases from approximately 300 microns to 160 microns. Many methods of producing thin crystalline silicon cells are being investigated. In the near future, wire saws may be able to create wafers as thin as 100 um routinely. A 100 um silicon wafer is very fragile and difficult to handle without fracture.
Typically, solar modules that are based on silicon wafers will undergo this general sequence of steps:    1. Fully metalize the cells. Screen printing of metal pastes and firing the pastes is commonly performed in the industry. Such screen printed pastes tend to produces lower efficiencies.    2. Test and bin the cells according to performance. Test probes are pushed onto the illuminated cell to measure actual performance. Similarly performing cells are binned together.    3. Solder leads to the cells. Referred to as “tabbing” the cells.    4. Solder cells together in series. Referred to as “stringing” the cells    5. Solder bypass diodes and external connection leads.    6. Transfer strings of soldered cells to a module. Referred to as “layup”. This entails transferring many cells soldered together and laying them on to an encapsulant layer on top of the front glass of the module.    7. Add backing encapsulant, back layer, and laminate.
Typical present-day module constructions for front contact cells and back contact cells are shown in cross section in FIGS. 1a and 1b respectively. The front glass 1 faces the sun. The silicon solar cells 4 have metal pieces 5 soldered to them. For front contact cells, the metal pieces must be soldered to the front of one cell and to the back of the next cell as shown in FIG. 1a. For back contact cells, the metal pieces 5 are soldered to the back of neighboring cells as shown in FIG. 1b. These cells and the metal pieces are sandwiched between two layers of encapsulant, front encapsulant 2 and rear encapsulant 3. The back sheet 6 is usually of a material with high resistance to moisture penetration to help protect the soldered pieces from corrosion. During lamination, the encapsulant layers 2, 3 soften and fill all gaps between the cells while also adhering the silicon wafers to the front glass and the rear backing layer.
Metallization is an ongoing issue with solar cells. Because screen printing and firing of metal pastes produces poor results, many alternative approaches have been pursued. Some can be quite complex and require many steps and alignments. Process complexity is at odds with low cost. Thus the key to success is to find approaches to reduce the number of steps and processes by inventing inexpensive processes which perform several functions at once. One example is Wenham et al, (U.S. Pat. No. 6,429,037) where a laser is used to simultaneously 1) drive a dopant into silicon, 2) open a dielectric layer, 3) and create a patterned surface for electroless metal plating. This is currently used for front contact cells. In this cell design, one polarity of the cell is contacted on the front side. While such solar cells are common, they have the drawback that the metal collection fingers on the front side shade portions of the solar cell, thereby causing efficiency losses. In order to electrically connect cells in series, conductors need to be soldered to the front side of one cell and then soldered to the back side of the next cell, increasing the handling and assembly complexity. A better approach is to put all the metallization on the back side of the cell. Such a design is called a back contact cell.
In a back contacted solar cell, a plurality of metal-semiconductor contacts, some anodes and some cathodes, are all on the backside of the solar cell. One advantage of the back contact cell design is that it avoids placing a metal contact grid on the front side of the solar cell which obscures part of the solar cell and reduces the absorbed light in the solar cell. Another advantage is ease in connecting cells since only back side to back side soldering is required. A disadvantage of the back contact cell design which has hindered more wide spread adoption, is that the metal on the backside must be patterned such that the two polarities are electrically isolated from each other. Additionally, it is preferred that all the metal of either polarity collect electricity to the fewest number of attachment points for external soldering.
One known pattern is that of interdigitated fingers which resemble two interpenetrating combs as in FIG. 2. The individual fingers of one polarity 7, 8 all connect to a common bus 9, 10 which is the soldering point for external electrical connections. The bus bars usually have portions that have large areas in order to ease the precision required in soldering metal conductors to the cells. This larger area, unfortunately, reduces the usable area of the wafer by not allowing collection fingers of the opposite polarity into the areas of the bond pads. In other words, the large area electrical busses 9, 10 obscure the collection of the opposite polarity carriers in the silicon underneath the busses. Additionally, the metal pattern must have low resistance which usually results in thicker metal layers. For silicon solar cells, thinner and more closely spaced metal fingers are advantageous to reduce losses and series resistance within the semiconductor. However, the fingers themselves must be of low resistance. Thus it is preferable to have fingers of high aspect ratio, of narrow widths and with narrow gaps between the fingers. This has been difficult to accomplish with few process steps and inexpensively.
One approach described by Mulligan et al. (published US application 2008/0210301) is to first deposit a thin blanket metal layer on the cell as an electroplating seed layer. A mask layer is then applied on top of the seed metal layer in a patterned manner. The exposed seed layer (regions without mask layer) is thickened by electroplating to create a thick interdigitated comb structure as in FIG. 2. The electroplating mask is then removed, and a short metal etch removes the exposed thin metal seed layer that was previously covered by the mask. What remains is an electrically isolated interdigitated comb structure such as in FIG. 2. This has a number of processing steps as well as inherent limits on the minimal distances between the fingers.
Hacke et al. (published US patent application 2008/0216887) has detailed several techniques to form and connect back metal patterns. More generally, a back contact solar cell has multiple rear points or regions of metal of either positive polarity or negative polarity. Two functions need to be accomplished for the module to operate. 1) all of the like polarities within a particular solar cell need to be electrically connected and 2) all the like polarities of one solar cell should be connected to the opposite polarity of an adjacent cell. Usually, the term ‘bus’ or ‘busbar’ refers to metal that performs task 1) and connects all like polarities of metal contacts on the solar cell. In this work, the solar cells have reduced area busbars, or are entirely busbarless, and current is extracted from several points on the interior of the cell surface. By moving the bus off of the wafer, the metal regions contacting the solar cell can cover more of the wafer and thus increase photogenerated carrier collection and the performance of the cell. The typical disadvantage of this approach is that multiple solder connections must be made between the common bus and each point or region of metal on the interior of the solar cell. More solder points raise the risk of solder failure.
In typical prior approaches, if soldering is required, then only certain metals can be used. Some metals are very difficult to solder to, such as aluminum, stainless steel, and chromium. Solderable metals include such metals as tin, copper, and lead. These metals also happen to be prone to corrosion by moisture. This becomes a module reliability issue and solder failure is one of the failure mechanisms of the module. Thus, in the module assembly, the rear backing layer(s) 6 has to block moisture penetration. Moisture barriers which can provide a solar module with twenty years of failure free operation tend to be expensive.
Another module failure mechanism occurs when the solder joint is mechanically weak. As the modules heat and cool in the day and night, the silicon and the connecting metal tabs expand and contract with differing expansion coefficients. A poorly soldered tab can partially or fully detach. This can increase the resistance in a small spot on the module and create “hot spots”. The result is a lowered module output and the hot spots can lead to other failures including delamination or glass breakage.
Soldering itself places local thermal and mechanical stresses on wafers. The solder temperature for lead-free solders can be around 230° C. Such localized temperatures combined with a slight applied pressure can cause fracture in silicon cells, especially as the cells become thinner and more fragile. This may not immediately result in an obvious fracture either. Instead, the cell may be laminated into a module where the fracture causes failure from the lamination stresses. Alternatively, years later, the fracture can grow due to thermal cycling and moisture and cause a significant reduction in the module output.
More generally, thin wafers of 160 um or less have difficulties tolerating the necessary handling of current fabrication processes including, metallization, testing, tabbing (soldering), stringing, and layup. Pick and place tools exert too much local pressure. Tiny flaws or cracks in thin wafers act as stress concentrators and allow for easy crack propagation. Furthermore, screen printing and metal plating can leave stresses that cause the wafer to bow. Thus one approach reported by Hieslmair et al. (published US patent applications 2007/0212510 and 2008/0202576) is to laminate the thin (35 um) silicon material to the front glass early in the process and leave the backside exposed for further cell processing. Back contact cell designs are advantageous for this approach since the fragile silicon is already supported by the front glass.